Danfoss VLT 6000 (Legacy Product) User guide

Type
User guide

Danfoss VLT 6000 (Legacy Product) is a versatile and efficient frequency converter designed to control AC motors in a wide range of industrial applications. With its advanced features and robust construction, it offers precise speed control, energy savings, and reliable operation. It is commonly used in variable torque applications such as fans, pumps, and compressors, and can also be used for constant torque applications with derating.

Danfoss VLT 6000 (Legacy Product) is a versatile and efficient frequency converter designed to control AC motors in a wide range of industrial applications. With its advanced features and robust construction, it offers precise speed control, energy savings, and reliable operation. It is commonly used in variable torque applications such as fans, pumps, and compressors, and can also be used for constant torque applications with derating.

VLT
®
6000 HVAC
MN.60.D1.02 - VLT is a registered Danfoss trademark
CHILLER
CHILLER
Fig. 1 - Traditional Primary/Secondary design
Pressure
Flow
100%
75%
86%
84%
Efficiency
curves
Design flow Flow 1
Impeller
curves
Pressure
Flow 1
Design - P2
P1
Design
flow
The Design
In Chilled Water systems, the Primary loop or
production loop, consists of pumps sized to handle
the chillers designed flow rate at a discharge pres-
sure just high enough to circulate the water through
the chiller and the rest of the primary piping loop.
This loop should be as small as possible, just large
enough to allow the secondary system to be
attached. This minimizes the resistance of the
primary loop and therefore the energy consumption
of the non-regulated constant flow pumps.
The primary pump flow is traditionally controlled by
throttling valves or balance valves on the discharge
of the pumps, figure 1. The pumps are often
oversized due to safety margin in the designs. By
adding losses to the pumping circuit with the
throttling valves, the proper design flow rate can be
established, figure 2 (moving from flow 1 to Design
flow).
Another method used is to trim the pumps impeller.
Once the system is operating, the balancing
contractor can determine the actual pressure drop
in the primary loop.
Once the actual pressure requirement of the pump
is established, the pumps impeller can be removed,
trimmed to the proper diameter, rebalanced, and
reinstalled in the pump.
Decreasing the diameter of the pumps impeller
reduces both the capacity and pressure of the
pump as desired, but also has an impact on the
pumps efficiency, figure 3.
The Application
Primary pumps in a primary/secondary pumping
system, such as shown in Figure 1, can be used to
maintain a constant flow through devices that
encounter operation or control difficulties when
exposed to variable flow. The primary/secondary
pumping technique decouples the primary
production loop from the secondary distribution
loop. This allows devices such as chillers to obtain
constant design flow and operate properly while
allowing the rest of the system to vary in flow.
Fig. 3 - Impeller trimming
Fig. 2 - Throttling valve
As the evaporator flow rate decreases in a chiller,
the chilled water begins to become over-chilled. As
this happens, the chiller attempts to decrease its
cooling capacity. If the flow rate drops far enough,
or too quickly, the chiller cannot shed its load
sufficiently and the chillers low evaporator
temperature safety trips the chiller requiring a
manual reset. This situation is common in large
installations especially when two or more chillers in
parallel are installed if primary/secondary pumping is
not utilized.
Primary system Secondary system
VLT
®
6000 HVAC
MN.60.D1.02 - VLT is a registered Danfoss trademark
CHILLER
F
CHILLER
F
Flowmeter
Flowmeter
The new standard:
Depending on the size of the system and the size of
the primary loop, the energy consumption of the
primary loop can become substantial.
A VLT frequency converter can be added to the
primary system, to replace the throttling valve and/or
trimming of the impellers, leading to reduced
operating expenses (fig. 4). Two control methods are
common:
The first method uses a flow meter. Because the
desired flow rate is known and is constant, a flow
meter can be installed at the discharge of each
chiller can be used to control the pump directly.
Using the built-in PID controller, the VLT frequency
converter will always maintain the appropriate flow
rate, even compensating for the changing resistance
in the primary piping loop as chillers and their pumps
are staged on and off.
Fig. 4
The VLT frequency converter solution
The other method is local speed determination. The
operator simple decreases the output frequency
until the design flow rate is achieved.
Using a VLT frequency converter to decrease the
pumps speed is very similar to trimming the pumps
impeller, except it doesnt require any labor and the
pumps efficiency remains higher, figure 5. The
balancing contractor simply decreases the speed of
the pump until the proper flow rate is achieved and
leaves the speed fixed. The pump will operate at
this speed any time the chiller is staged on.
Because the primary loop doesnt have control
valves or other devices that can cause the system
curve to change and the variance due to staging
pumps and chillers on and off is usually small, this
fixed speed will remain appropriate. In the event the
flow rate needs to be increased later in the systems
life, the VLT frequen-cy converter can simply
increase the pumps speed instead of requiring a
new pump impeller.
Fig. 5
Pump efficiencies with variable speed
(Figure courtesy of ITT Bell & Gossett)
P
VLT
Flow
Figure 5 shows that the pumps efficiency remains
constant as the speed is reduced to obtain the
design flow. This differs from the results of the
trimming the impeller where the efficiency de-
creases (fig. 3).
VLT
®
6000 HVAC
!
MN.60.D1.02 - VLT is a registered Danfoss trademark
Energy saving calculation example
In the following calculation example, a 30 kW
primary pump´s operation cost with a VLT frequency
converter is compared to the same pumps opera-
tion cost with a throttling valve.
For the purposes of a comparison, the pumps
original design discharge pressure is assumed to be
15% overheaded.
Annual operation load profile
Since primary pumps by definition are constant flow,
the flow rate is always at 100%. What the profile
doesnt show is the discharge pressure required.
Example:
A 30 kW primary pump is operating by a constant
flow at 100% for 24 hours every day.
Total annual operating hours is: 24h x 365 days = 8760 hours.
Energy consumption per year: 30 kW x 8760h = 262800 kWh
Energy saved by using a VLT: 15% of 262800 kWh = 39420 kWh
Annual money saved by using a VLT: 39420 kWh x USS 0,10 = USS 3942
Simple payback: 1.5-2.5 years
It is normal for pumps to be overheaded by 15% or
more to compensate for possible installation varia-
tions and other safety calculations. For constant
flow applications, this overpressurization causes a
proportional waste of energy. Using a VLT frequency
converter to balance the system instead of throttling
valves allows for the elimination of the otherwise
wasted energy.
Sensor Type And Placement
For Primary Pumping systems, the sensor location is
not as critical since a flowmeter is not really required,
but when used, proper location will avoid any
operational difiiculties. The proper location for a flow
meter is at the discharge or inlet of each chiller,
assuming each chiller has a dedicated pump and
VLT frequency converter.
This allows the VLT frequency converter system to
guarantee the design flow rate through each chiller
and take advantage of the decreased resistance in
the piping network as parallel chillers are destaged.
An example schematic showing the sensor
placement is shown in Fig 4.
Differential pressure sensors can also be located
across each chiller because differential pressure
across a non modulating load also measures flow.
Since flow is the critical parameter however, it is
recommended that a flow meter be utilized.
Drive price
Energy savings
VLT
®
6000 HVAC
"
MN.60.D1.02 - VLT is a registered Danfoss trademark
Comparison of installation and maintenance
costs
Aside from the potential energy savings,the cost of
using a VLT frequency converter can be partially
paid for by the savings on installation and main-
tenance costs. The traditional system not only
requires the throttling valves and balancing valves,
but also needs at a minimum: 6-wire motor cable, a
softstarter, and power factor correction capacitors
(PFCC´s). When opting to trim the pumps impeller,
system down-time and a considerable amount of
labor is also saved.
Maintenance is limited to a minimum, and instal-
lation costs and space can be saved. The VLT
frequency converters IP 54 enclosure allows it to be
mounted in the mechanical equipement room close
to the pumps on the wall.
A smart design combined with the falling costs of
electronic equipment and increasing costs in labor
have brought the investments for most regulation
methods to a similar level. The comparison of the
energy consumption, installation costs, and de-
creased maintenance costs show why the utilization
of VLT frequency converters are the best choice.
Utilizing the Danfoss VLT frequency converter
makes the valves, soft-starters, power factor
corrections, and extensive cablework unnecessary.
Manual speed adjustment or a simple voltage
(0...10V) or current (0/4...20 mA) control signal and
flow meter is sufficient to vary the flow.
True pay-back calculation:
Cost of drive + flow meter - (throttling valves + starter + wiring + PFCC´s) - saving in commissioning time
Energy savings + annual maintenance savings
0 year ± 0.5 year
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Danfoss VLT 6000 (Legacy Product) User guide

Type
User guide

Danfoss VLT 6000 (Legacy Product) is a versatile and efficient frequency converter designed to control AC motors in a wide range of industrial applications. With its advanced features and robust construction, it offers precise speed control, energy savings, and reliable operation. It is commonly used in variable torque applications such as fans, pumps, and compressors, and can also be used for constant torque applications with derating.

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